Estrogen Receptor Alpha as a Mediator of Life-History Trade-offs

farianto;

Trade-offs between competitive and parental strategies often are mediated by sex steroids. The mechanisms underlying steroid signaling and metabolism may therefore serve as targets of disruptive selection that leads to alternative behavioral phenotypes. White-throated sparrows exhibit two color morphs that differ in both competitive and parental behavior; white-striped (WS) birds engage in more territorial singing, whereas tan-striped (TS) birds provision nestlings more often. Although WS birds have higher levels of plasma testosterone (T) and estradiol than do TS birds, experimental equalization of these hormones does not abolish morph differences in singing. Neural sensitivity to sex steroids may differ between the morphs because the gene for estrogen receptor alpha (ERa) has been captured by a chromosomal rearrangement found only in the WS birds. We recently showed that expression of this gene differs between the morphs
and may drive the behavioral polymorphism. First, the ERa promoter region contains fixed polymorphisms that affect transcription efficiency in vitro. Second, in a free-living population, local expression of ERa depends strongly on morph and predicts both territorial singing and parental provisioning. Differential ERa expression is particularly striking in the medial amygdala; WS birds have three times more ERa mRNA than do TS birds. This difference persists during the nonbreeding
season and is unaffected by exogenous T treatment. Finally, preliminary data generated by RNA-seq confirm that ERa expression in MeA is both differentially expressed and correlated with territorial singing. Together, these results suggest that ERa may be a target of disruptive selection that leads to alternative behavioral strategies. Our future directions include a more detailed analysis of the ERa promoter regions to determine the molecular basis of differential
expression as well as gene network analyses to identify genes connected to ERa.